S1, the first heart sound in the cardiac cycle, occurring at the outset of ventricular systole. It is associated with closure of the mitral and tricuspid valves and is synchronous with the apical pulse. Auscultated at the apex, it is longer, and lower than the second sound (S2), which follows it. The sound of the mitral valve is loudest at the apex of the heart, and that of the tricuspid valve is loudest at the left sternal border in the fourth intercostal space. See also lubb.
